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So yeah, we don't want to deprecate string.Template. It has a very specific use case that's used a lot, i.e. making strings dead simple to translate. %(foo)s was very problematic. {foo} is a little better, but looks too weird for most translators. $foo is very common, well understood, and hard to get wrong.
As for modernizing the code (the L146 bit was thanks to Python 2), I wouldn't be against it if it doesn't change the documented API or functionality and all the tests still pass (and there's good coverage - I don't remember anymore). I don't think it's really that important though; typically these are not used in performance critical sections.
Re: ${thing.attribute} - no, that wouldn't keep them Simple and PEP 292 was deliberately targeting simplicity.
Bottom line: keep $-strings simple and focused on their original use case. For more complicated use cases, use str.format().